1. Field of the Invention
The present invention relates to an endoscope.
2. Description of the Related Art
In an endoscope which includes an image pick-up unit at a front end portion, and which is provided with a flexible tube having a bent portion, generally, for observing an endoscope image in an optimum state, it is necessary to adjust a focal point by moving a focusing lens and an image pick-up element.
As an endoscope apparatus for solving this problem, an image pick-up apparatus for endoscope described in Japanese Patent Application Laid-open Publication No. 2004-129950 has been proposed. In the image pick-up apparatus for endoscope according to this proposal, at a front end portion of the endoscope, a shape memory alloy is let to be a variable means of relative positions of an optical element and an image pick-up element.
However, when a shape memory element is used in an actuator which drives a lens as in a conventional technology, generally, when the shape memory alloy is held for a long time with a load exerted thereon, in a heated state, a lattice defect is developed inside, and a function as an actuator is declined.
Since this developing becomes remarkable when a shape memory treatment having a substantial displacement due to heating is carried out, there is a problem that it has been difficult to achieve a substantial displacement for using stably for a long period of time.
Moreover, for restricting a movable range of a lens accurately, it is desirable to restrain mechanically the movable range of the lens. However, in such structure in which a movement margin is taken into consideration, a substantial stress is exerted on a shape memory alloy at the time of heating. Therefore, the abovementioned problem is particularly remarkable.